March 12, 2013

Emissions Models Underreporting Ozone Reductions, Claim Researchers

Ground-level ozone may be falling even faster than popular prediction models have been reporting, according to new research conducted by environmental experts at Rice University in Houston, Texas.

The study suggests that recent pollution controls designed to cut down on industrial and vehicular emissions may be doing a better job of lowering dangerous ozone levels than even the Community Multiscale Air Quality (CMAQ) model has calculated.

In fact, the CMAQ may have misjudged ozone level reductions by 20 to 60 percent, the authors claim in a paper published online by the journal Atmospheric Environment. The effect is especially prevalent in the northeast, where the effect has surpassed what had been expected following cuts to nitrogen oxide (NOx) emissions between 2002 and 2006.

“The models have been underpredicting how much benefit we get from controlling NOx emissions in some instances,” said Daniel Cohan, an assistant professor of civil and environmental engineering at the university. Cohan authored the study along with Wei Zhou, a Rice graduate student, and Dr. Sergey Napelenok, a scientist working in the US Environmental Protection Agency´s (EPA) Atmospheric Modeling and Analysis Division.

“Following major controls of NOx, ozone has come down more quickly than anticipated,” Cohan added. “This is good news. But it also poses a challenge because states rely upon models to predict whether they´ll attain ozone standards in the future. If the models have key uncertainties that affect their responsiveness, that can affect the states´ control strategies.”

Rather than being emitted directly, ozone actually forms from NOx and hydrocarbon emissions that combine close to ground level. In order to help states comply with EPA standards for ozone — which are currently at 75 parts per billion (ppb) — modeling of the complex chemistry behind the process is required, the researchers explain.

More than a decade ago, the EPA mandated a cap-and-trade program known as the NOx SIP Call, which was designed to lower emissions of the ozone-forming compound from states in the eastern US.

During the first four years following the program´s implementation, the emission levels served as a sort of real-world experiment that allowed the Rice University team to determine exactly how accurate computer modeling was at predicting improving air quality conditions. As it turns out, the simulated drop in ozone was 4.6 ppb, but observational research put the levels at 8 ppb — a significant difference, according to the researchers.

“How ozone responds to changes in NOx and hydrocarbons is a nonlinear chemistry,” Cohan said. “So it´s certainly possible that even the best models could be slightly inaccurate in defining those relationships. It tells us that, as modelers, we need to revisit the formulations, especially the chemistry.”

“The goal of everyone in the process is to reach attainment in the most cost-effective manner possible, and we need accurate models to inform those decisions,” he added.